The Norway rat (Rattus norvegicus) is globally widespread and costs primary industry hundreds of millions of dollars per year. It has caused or contributed to the extinction or range reduction of native mammals, birds, reptiles and invertebrates through predation and competition. It restricts the regeneration of many plant species by eating seeds and seedlings, eats food crops and spoils human food stores by urinating and defecating in them. Additional economic damage is caused by chewing through power cables and spreading diseases.

DescriptionThe Norway rat has brown fur on the back with pale grey fur on its belly. The adults normally weigh 150 - 300g, and may reach up to 500g, and are up to 390mm long. They have relatively small ears - which usually do not cover the eyes when pulled forward. The tail is shorter than the head-body length - the opposite is true for the ship rat R. rattus (Wittenberg, R. (ed.) 2005). Females have 12 nipples.

Habitat descriptionNorway rats can be widespread, utilising most habitat types, but they appear to show a preference for wetland habitats. The home range of the Norway rat averaged 5.8ha for males and 5.1ha for females, according to the results from a small study on Kapiti Island off New Zealand (Bramley, 1999; in Innes, 2001). In the UK, male rats had a mean range length of 678m, with that of females being smaller (Macdonald et al., 1999; in Innes, 2001). In Europe, the Norway rat exists primarily in close relationship with humans, but there are also ‘wild’ populations along water edges. The Norway rat is considered to be territorial throughout most of the year, but they will spread when food is scarce, and migrations have been observed (Wittenberg, R. (ed.) 2005). Norway rats rarely climb trees. In the Galapagos Islands, they prefer to move along underground cracks and crevices in the lava rocks (Key and Woods, 1996; in Innes, 2001). From the distribution and recorded reinvasions of Norway rats it appears that they can cross up to 1km of water comfortably, and up to 2km of open water more rarely when conditions are suitable (mudflats, intermediate rocky islets, tidal flow, etc.) (Russell and Clout, 2005).

General impactsNorway rats are known to restrict the regeneration of many plant species by eating seeds and seedlings. They prey upon most animal species smaller than themselves such as reptiles, small birds, birds eggs and freshwater and intertidal species. Norway rats eat food crops and spoil human food stores by urinating and defecating in them. Additional economic damage is caused by rats chewing through power cables etc. and spreading diseases.

Both R. norvegicus and Rattus rattus transmit the plague bacterium (Yersinia pestis) via fleas in certain areas of the world. There have been a series of recent outbreaks in Madagascar in recent years (Boiser et al. 2002).

Geographical rangeNative range: Norway rats are believed to have originated in NE China. Known introduced range: Rattus norvegicus were not known to be present in western Europe until 1700-1716. They are now present in the United Kingdom, France, Switzerland, Portugal, Denmark, Belgium, USA, Mexico, Canada, Chile, Caribbean, Galapagos Islands, Mauritius, Seychelles, La Réunion, Japan, New Zealand, Falkland Islands, Pacific Islands, South Georgia and South Sandwich Islands, US minor outlying islands, and French Southern Territories. There is ongoing risk of invasion of new islands.

Introduction pathways to new locationsSeafreight (container/bulk):Rattus norvegicus can be transported in either bulk or loose equipment or simply by stowing away on a vessel. Their habit of living near wharves increases the chances of this happening.

Management informationPreventative measures: Research has shown that it can often be difficult to eradicate rats from islands in the early stages of invasion, hence it is better to prevent rodents arriving on islands in the first place. Eliminating a single invading rat can be disproportionately difficult because of atypical behaviour by the rat in the absence of conspecifics, and because bait can be less effective in the absence of competition for food (Russell et al., 2005). Weihong et al. (1999) provide useful information regarding the detection of rodent species using different trapping methods and bait, Dilks and Towns (2002) published by New Zealand’s Department of Conservation discusses how to detect and respond to rodent invasions on islands.

Physical: Trapping is often used on a local scale, however it generally fails to remove all individuals, as trap-shy animals can survive and repopulate the island (DoC, 2004).

Chemical: Use of anticoagulant poisons is the most common method of control. On islands, eradications have been achieved by the use of poisons. However, strict quarantine is required to prevent further spread of this species to additional islands. One of the world's largest successful eradication operations was on the 3,100 hectare Langara Island in British Columbia, Canada. The eradication campaign was begun (after preparation and trials) in July 1995 and the island was declared free of rats in May 1997 (Kaiser et al., 1997). Another example of a successful rat eradication was on Kapiti Island, New Zealand (1970 ha) where "second-generation" anticoagulant poisons have been used (Empson and Miskelly, 1999). The world's largest rat eradication project to date is on Campbell Island (11,300 ha), where eradication was declared in 2003.

Fisher et al. (2004) suggest that diphacinone especially, and also coumatetralyl and warfarin, should be evaluated in field studies as alternative rodenticides in New Zealand. Brodifacoum, the most widely used rodenticide in New Zealand currently, can acquire persistent residues in non-target wildlife. Mineau et al. (2004) discussed a risk assessment of second generation rodenticides at the 2nd National Invasive Rodent Summit. O'Connor and Eason (2000) discusses the variety of baits which are available for use on offshore islands in New Zealand.

An investigation Spurr et al. (2007) was carried out to assess the behavioural response of ship rats to four different bait station types. Yellow plastic pipe, wooden box (‘rat motel’), and wooden tunnel bait stations were found all suitable for surveillance of ship rats and the first two at least for Norway rats (all were readily entered and had a similar amount of bait eaten from them).

Biological: Contraceptive methods of control are currently experimental, but the potential for effective control using contraceptive methods is promising. National Wildlife Research Center (USA) scientists are working on several possible formulations that may make effective oral immunisation possible (Nash and Miller, 2004).

NutritionOmnivorous and opportunistic - including raw or cooked meat and vegetable matter, grains and other seeds and berries as well as roots and a wide variety of vertebrate and invertebrate species. Adults require about 10% of their body weight per day in dry grain, and when on a dry diet they need to drink about 25ml of water. R. norvegicus in captivity has been observed to withdraw food to the nest, and sometimes store it there (Barnett and Spencer, 1951; in Campbell et al., 1984).

ReproductionPlacental, sexual. Females are polyestrous and ovulate spontaneously. Breeding largely determined by food availability. Litter size normally 6 - 11, gestation is 21-24 days, young weaned at about 28 days. Females can be sexually active in the season of their birth.

Lifecycle stagesOn Fregate Island in the Seychelles, juvenile rats first ventured from the den when they were 30-50g in weight (Thorsen et al., 2000; in Innes, 2001).

Compiled by: IUCN SSC Invasive Species Specialist Group Updates with support from the Overseas Territories Environmental Programme (OTEP) project XOT603, a joint project with the Cayman Islands Government - Department of Environment

The Global Invasive Species Database is managed by the
Invasive Species Specialist Group (ISSG) of the IUCN Species
Survival Commission. It was developed as part of the global
initiative on invasive species led by the Global Invasive
Species Programme (GISP) and is supported through partnerships
with the National Biological Information Infrastructure,
Manaaki Whenua-Landcare Research and the University of
Auckland. Conditions of use.